Method for locating a radio tag

ABSTRACT

The invention further relates to a radio tag with a communication stage and a logic stage that interacts therewith The invention relates to a method for locating a radio tag whose position is unknown, wherein in a group of radio tags, in particular designed as electronic price-indicating devices, a locating signal is either a) emitted by one or more radio tags whose position is known and received by the radio tag whose position is unknown or b) emitted by the radio tag whose position is unknown and received by one or more radio tags whose position is known, and, in both cases a) and b), the reception quality for the locating signal is determined and provided, for the radio tag receiving the locating signal, as a basis for narrowing down the position of the radio tag whose position is unknown.

TECHNICAL FIELD

The invention relates to a method for locating a radio tag.

The invention further relates to a radio tag with a communication stageand a logic stage that interacts therewith.

The invention further relates to a system for locating a radio tag.

BACKGROUND

Conventional systems and methods for locating a device designed toreceive and/or transmit radio signals are mostly based on triangulationor runtime measurement methods. However, for a wide variety of reasons,such methods have proven to be disadvantageous when used inside abuilding or room, and only have limited application, if any. Since anyinterior space, in particular each business premises, is unique and initself exhibits a high level of inhomogeneity with respect to radiosignal propagation, position determination, for example viatriangulation, using several fixedly installed access points has provento be inexpedient.

The object of the invention is to provide a method, a radio tag, as wellas a system, so as to avoid the problems mentioned at the outset.

SUMMARY OF THE INVENTION

This object is achieved by a method according to claim 1, a radio tagaccording to claim 15, and a system according to claim 24.

Therefore, the subject matter of the invention relates to a method forlocating a radio tag whose position is unknown, wherein in a group ofradio tags, in particular designed as electronic price-indicatingdevices, a locating signal is either a) transmitted by one or more radiotags whose position is known and received by the radio tag whoseposition is unknown, or b) transmitted by the radio tag whose positionis unknown and received by one or more radio tags whose position isknown, and, in both cases a) and b), the reception quality of thelocating signal is determined and provided at the radio tag receivingthe locating signal in order to narrow down the position of the radiotag whose position is unknown.

The subject matter of the invention further relates to a radio tag witha radio communication stage for receiving a locating signal transmittedby another radio tag and a logic stage that interacts with the radiocommunication stage, wherein the logic stage is designed to enablereception of the locating signal as soon as the other radio tagtransmits the locating signal, and wherein the radio tag (Tn) isdesigned to evaluate the received locating signal with respect to thereception quality of the locating signal and to provide the determinedreception quality for narrowing down the position of a radio tag whoseposition is unknown.

The subject matter of the invention further relates to a system forlocating a radio tag whose position is unknown, comprising a group ofradio tags, wherein either a) one or more radio tags whose position isknown are designed to transmit a locating signal and the radio tag whoseposition is unknown is designed so that the locating signal can bereceived as soon as the radio tag(s) whose position is known has/havetransmitted it, or b) the radio tag whose position is unknown isdesigned to transmit a locating signal, and one or more radio tag(s)whose position is known are designed so that the locating signal can bereceived as soon as the radio tag whose position is unknown hastransmitted it, and in both cases a) and b), the radio tag that receivesthe locating signal is designed to evaluate the locating signal withrespect to the reception quality of the locating signal and to providethe determined reception quality.

The provided reception quality can be used to narrow down or evendetermine the position of the radio tag whose position is unknown. Onlya single access point is here necessary, to which the tags involved inthe search are allocated by radio. If a system consists of severalaccess points and tags individually allocated thereto, these accesspoints and the tags allocated thereto can uninterruptedly handle anormal communication operation, while only a single access point withthe tags allocated thereto is involved in the search for the tag alsoallocated and to be allocated thereto. This minimizes interference inoverall system performance.

A radio tag (generally also referred to as “radio label”), hereinafterabbreviated to tag, essentially comprises a radio communication stage,also called a transceiver, and a logic stage that interacts therewith,which provides the logical function of a tag. For example, the logicstage can be realized completely by hardware, or exhibit amicroprocessor and memory chips or a microcontroller with integratedmemory chips, making it possible to run software stored in the memorychips. A tag can receive a radio signal with its radio communicationstage, process the reception data contained in the radio signal with thelogic stage, and, if needed, generate response data with the logic stageand transmit them again as a radio signal via the radio communicationstage. The radio communication stage exhibits means for radiocommunication and the conversion of analog signals into digital signalsand vice versa.

For example, the radio protocol can be implemented according to the“ZigBee” standard, the “BlueTooth” standard or even according to aproprietary protocol. The logic stage implements the radio protocol, sothat communication can be carried out between the tag and access pointbased on the systematics and timing of the radio protocol.

For example, such a tag can be integrated into an electronicprice-indicating sign (known in the jargon as “electronic shelf label”,abbreviated as ESL). However, the tag can also be a constituent ofanother device, or present as a self-contained device, for exampleencapsulated in its own housing.

In a preferred exemplary embodiment, use is made of a proprietary radioprotocol that implements a time slot communication method, by means ofwhich several tags can communicate with an access point. An access point(generally also referred to as “communication device”) is a base stationthat serves as an interface between wired communication, e.g., with adata processing device (e.g., a server), and wireless communication withother devices, in the present case the tag. According to this protocol,the tags can first be registered at the access point or be allocatedthereto, so that they can be used for communicating with this accesspoint. During registration, each tag has assigned to it a predefined,individual time slot for communicating with the access point. Forexample, within a period of n seconds, e.g., 15 seconds, m time slots,e.g., 255 time slots, are here used. The n seconds constitute a timeslot cycle, which continuously repeats and is also referred to as asynchronization cycle. In this time slot communication process, m timeslots are thus available within a synchronization cycle forcommunicating with tags. Each of the tags is allocated to one of thetime slots, wherein a specific time slot can also have several tagsallocated to it, e.g., 2, 3 or 4 tags. Four synchronization cycles eachwith 255 time slots exist within one minute, so that, for example, 2tags per time slot can be used to address a total number of 2040 tags.

For purposes of energy supply, such a radio tag can comprise an energystorage device, e.g., a battery or a solar panel coupled with arechargeable battery. To operate as energy efficiently as possible, thetags exhibit various operating states.

The tags are operated in a normal mode. In this normal mode, they areeither in an active state with a relatively high energy consumption, orin a sleep state with relatively low energy consumption. The activestate is present in a time slot intended for it for communicating withthe access point. In the active state, it exhibits a reception readinessso as to receive commands and potentially also received data from theaccess point, and process the latter via the logic stage. In the activestate, the logic stage can also be used to generate transmission dataand communicate them to the access point. Outside of the time slotintended for them, the tags are operated in an energy-saving sleepstate. In the sleep state, the logic stage only performs thoseactivities required for timing purposes to wake up on time just prior tothe respective time slot allocated to the tag, so that it is ready forcommunicating with the access point in the next time slot intended forit. During communication, a data packet comprising data abouttransmitter identification, receiver identification, control and/orinformation content can be used, as can data about reception quality orapplication.

With the assistance of a synchronization signal structure (e.g., arelative short signal at the start of the respective time slot) sent outvia the access point in each of the time slots, the radio tags cansynchronize themselves for the first time with the time grid of the timeslot communication method, so as to start the communication operationwith the access point, re-synchronize it during operation should it havelost the synchronous state for whatever reason, and keep it synchronousgiven the absence of any smaller deviations from the synchronous statecaused by inaccuracies, such as a drift of its internal clock.

For example, the locating signal can comprise a fixed, predefined datasignal structure and/or comprise a predefined data content. However, thelocating signal can also comprise varying structures or a variable datacontent on a case to case basis. Any data contents, e.g., random datacontents, can also be used if it has been determined in the systembeforehand that a locating signal must now be expected. The locatingsignal can also contain the identification of the respectivetransmitting tag, or only be defined by this identification.

A radio tag, in particular its radio communication stage, can bedesigned so that the tag can itself determine or measure or evencalculate the relative reception field strength of a received radiosignal. A so-called “received signal strength indicator” (RSSI) is hereprovided in the tag, and represents an indicator for the reception fieldstrength. Since the RSSI has no fixed unit, the value of the RSSI mustbe interpreted depending on the data sheet of the manufacturer, whereina higher value for the RSSI usually denotes a higher reception fieldstrength. Already the value of the RSSI taken by itself can serve as agauge for reception quality. In addition, the tag, in particular itsradio communication module or the logic stage, can be designed todetermine the quality of data transmission for a received signal. Aso-called “link quality” (LQ) is here determined in the tag. It may beadvantageous to also consider the link quality in determining thereception quality, because aside from the RSSI, it also considers thequality of the data, i.e., the usability of the information content ofthe received radio signal. In a preferred embodiment, both values areweighted for determining the reception quality in such a way that thesum of the weightings yields 100%, meaning for example that the RSSIcontributes 80% to the reception quality, and the LQ 20%. However, otherweightings can also be used, e.g., 35% RSSI and 65% LQ or 50% RSSI and50% LQ. However, a value for “link quality” taken by itself can alsoserve as a gauge for indicating reception quality.

The advantage to the invention is that a radio tag that is missing or tobe localized can be found using one or more other tags. If only oneother tag is used, its position can at least be narrowed down. However,several other tags whose position is known are preferably used. The moreother tags are used, the more precisely the position of the tag to belocalized can be determined. Therefore, the tag whose position isunknown can be found especially well using a swarm of other tags. Asopposed to conventional methods, for example which require a specialconfiguration of the access point or a special number of access points(e.g., three for triangulation) or a special positioning of the accesspoints for ascertaining or narrowing down the position of a tag, onlytags whose position is known are themselves used as position referencesin the present case. The plurality of small nodes (tag swarms)distributed in the room thus resolves a problem in a fast and efficientmanner, which either can only be inadequately resolved with a number (2,3 or 4) of access points somewhere in the room (e.g., centrallypositioned), if at all. According to the invention, the significance ofa distance from the respective transmitting tag is attributed to therespective reception quality determined on a receiving tag, from which,knowing the position of tags whose position is known, the position ofthe tag being searched for or to be tracked (whose position is unknown)can be determined or narrowed down.

The invention can be used effectively especially in a business premises,i.e., inside of a building, in which the entire business area is coveredby radio with a few, e.g., only two, access points, and the radio rangesof the access points overlap over large areas. Up to 20,000 tags or morecan there be localized in varying positions (e.g., on shelves orfastened to products). If, say, five of this large number are not intheir predefined location, these missing tags are virtually impossibleto find. The invention here solves the problem, offering a compact andefficient solution to finding or searching for or even tracking a singletag.

In all variants of the invention discussed below, a tag thus receives acommunication of another tag, which is intended either for the receivingtag itself or for another receiver (e.g., the access point), andevaluated at the receiving tag with respect to its reception quality,yielding the basis for position localization. The accuracy rises withthe number of tags participating in the search.

Additional, especially advantageous embodiments and further developmentsof the invention may be gleaned from the dependent claims and followingdescription. The radio tag or even the system can here be furtherdeveloped to reflect the dependent claims for the method. Advantagesdiscussed in conjunction with features of one category or its claimsalso apply analogously to any other category or their claims.

A communication, e.g., between the radio tags or a radio tag and theaccess point, can take place bidirectionally or unidirectionally, oreven encrypted, for purposes of localizing the position of the tag whoseposition is unknown or during normal communication operations.

In a first embodiment of the invention, the radio tag whose position isknown and that received the locating signal transmits the evaluatedreception quality to a data processing device, and the data processingdevice narrows down the position of the radio tag whose position isunknown, knowing the position of the radio tag(s) whose position isknown and taking into account the reception quality received by therespective radio tag whose position is known. This measure can be usedfor a first application.

In this first exemplary application, let it be assumed that the tagwhose position is unknown was instructed by the access point to transmitlocating signals. Tags whose position is known were instructed by theaccess point to listen for them. The locating signals can be transmittedand received in a time window detached from the time slot communicationmethod. However, they can also be transmitted in those time slots of thetime slot communication method that are allocated to the respective tagswhose position is known. The locating signals can be addressed orunaddressed. Each involved tag whose position is known registers eachreceived locating signal and remembers (stores) the determined receptionquality. In the present case, a data pair comprised of their ownidentifier and reception quality need not be stored; rather, it issufficient to store the determined reception quality. This is because,in order to localize the position, the access point of each tag whoseposition is known is used to query the previously determined receptionquality, wherein the access point knows with certainty from which tagwhose position is known this happens. The access point, which knows theidentifier of the tag participating in the communication, thus assemblesthe data pair on its side from the identifier of the tag whose positionis known and that was just queried and the reception quality obtained bythis tag, and delivers this data pair to the data processing device,where the final evaluation for localizing the position of the tag whoseposition is unknown takes place. Advantageous here is the relatively lowdata volume when transmitting the relevant data (in the present case,those data that only represent the reception quality) from the tag whoseposition is known to the access point.

In the first application, however, a data pair can also be generatedfrom its own identifier and determined reception quality in the affectedtag whose position is known, so as to communicate this data pair to theaccess point.

Within the framework of the first application, implementation can alsoinvolve having the tag whose position is known evaluate and provide thereceived locating signal in terms of the reception quality as well asthe identifier of the transmitting tag whose position is unknown, i.e.,transmit the latter to the access point for further processing by thedata processing device. This is advantageous when simultaneouslysearching for several tags whose position is unknown.

In this first application, the values for reception quality required forlocalizing the position of the tag whose position is unknown,potentially also present as data pairs with identifiers, are availablestored on individual tags whose position is known, as explained above,and can there be individually queried, i.e., collected for furtherprocessing. If the search takes place detached from the time slotcommunication method, the results of the evaluation (receptionquality/data pairs) can be transmitted to the tag whose position isunknown in individual time slots after re-synchronizing the tagsparticipating in the search, centrally collected there, and from therealso be transmitted via the access point to a data processing device.However, they are preferably transmitted by the respective tags whoseposition is known directly via the access point to aforesaid dataprocessing device.

In a second embodiment of the invention, the radio tag whose position isknown that transmits the locating signal transmits the locating signalalong with its identifier, and the radio tag whose position is unknownthat receives the locating signal reevaluates the locating signal withrespect to the identifier transmitted along with the locating signal,and provides a data pair comprised of the identifier and accompanyingreception quality. This measure can be used for a second and thirdapplication.

In the second exemplary application, let it be assumed that the tagwhose position is unknown was instructed by the access point to listenfor locating signals. Tags whose position is known were instructed bythe access point to transmit locating signals addressed to the tag whoseposition is unknown. The tag whose position is unknown registers eachreceived locating signal addressed to it with the help of the aforesaiddata pair. Addressing can take place explicitly in the signal byindicating the identifier of the tag whose position is unknown, orimplicitly arise from the behavior (transmitter or receiver state duringa specific timespan) of the participating tags.

In the third exemplary embodiment, let it be assumed that the tag whoseposition is unknown was instructed by the access point to listen forlocating signals. Tags whose position is known were instructed by theaccess point to transmit locating signals addressed to the access point.These locating signals can, but do not have to be regular data signalsthat are communicated by the tag whose position is known to the accesspoint. While the tag whose position is known communicates data to theaccess point, the tag whose position is unknown listens in on this datasignaling traffic to the access point. For example, this can take placeby having the tag whose position is unknown be awake not just in itstime slot and listening for signals, but also be awake in those timeslots and listening for signals allocated to the other tags whoseposition is known. However, a timespan for this case can also be definedoutside of the timing for the time slot communication method. Tagsparticipating in the search must then exit the time slot communicationmethod to receive the locating signals. The tag whose position isunknown registers each received locating signal addressed to the accesspoint using the aforesaid data pair.

In both the second and third applications, the tag whose position isunknown listens for locating signals from the swarm or group of othertags whose position is known, and centrally accumulates a number of datapairs for subsequent evaluation.

Within the framework of the second embodiment of the method, the datapair could be evaluated for determining or narrowing down the positionof the tag whose position is unknown, for example within the tag whoseposition is unknown. To this end, the tag whose position is unknown mustknow the positions of the other tags whose position is known, or atleast know a correlation between the reception quality and distance. Itcan have obtained this knowledge by previously downloading correspondingdata, e.g., position data for tags whose position is known, from a dataprocessing device. To determine its position, the tag whose position isunknown converts the reception quality into a distance from therespective tag identified by the identifier, and can roughly narrow downor even determine relatively precisely its position depending on thenumber of received locating signals. The distance determined by therespective tag identified by the identifier can still be subjected to acorrection, wherein this correction takes into account the environmentof the respective tag, and thus the signal propagation characteristicsin its environment. The position determined in this way is thentransmitted to the data processing device. However, the position of thetag whose position is unknown can also be determined relative to the tagwhose position is known.

However, each radio tag whose position is unknown that receives thelocating signal preferably transmits the data pair to a data processingdevice. This is to be preferred, since the computationally intensive andultimately energy consuming activities of the logic stage are avoided asa result. All data pairs or just a selection thereof corresponding to acriterion can be transmitted. For example, the criterion used can be thereception sequence or ranking based on reception quality. Only the fiveor ten best ranked data pairs can then be transmitted, for example,which brings with it a significantly improved energy efficiency than ifall data pairs available were to be transmitted. System efficiency isalso improved significantly, because transmission takes place faster,and the system returns to the normal mode earlier than would be the casegiven a complete transmission of all data pairs. After reception of thedata pair(s), the data processing device narrows down the position ofthe radio tag whose position is unknown, knowing the position of theradio tag whose position is known and taking into account the data pairsreceived by the radio tag whose position is unknown. Even though theposition of the tag whose position is unknown continues to be determinedbased on monitoring the communication between a tag and its environment,while activities that require memory resources and computational powerare outsourced to the data processing device. After one or more datapairs have been received, the data processing device has at its disposalall additional information for determining the position of the tag whoseposition is unknown, for example the position coordinates of the tagwhose position is known, the correlation between the reception qualityand distance for the respective tag, and potentially also the correctionto be applied in considering the environment of the respective tag. Forexample, if aforesaid used tag is integrated in an electronicprice-indicating sign, the data processing device has at its disposalexact coordinates for tags whose position is known, since each pricesign is allocated to precisely one product, and the position of theproduct in the store is notated in a so-called planogram down to itsexact location in a shelf. Proceeding in this way also ensures that theenergy reserves of the tag whose position is unknown are conserved asmuch as possible. If no planogram is available, known coordinates of thetag whose position is known and/or relative distances between the tagswhose position is known can be used to localize the unknown tag.

In another aspect of the invention, the radio tag intended fortransmitting the locating signal, in particular the tag whose positionis known, is switched from its normal mode into a search mode by areceived search command, wherein the locating signal and/or the time fortransmitting the locating signal and/or the frequency of transmittingthe locating signal is determined in the mode. The switch is made bydecoding the search command by means of the logic stage.

A timing that deviates from the time slot communication method used inthe normal mode can be applied as the time for transmitting the locatingsignal. Transmission can here be started as soon as the last searchcommand was delivered to an involved tag. The start can here take placeupon expiration of the respective time slot cycle, or also before therespective time slot cycle has even completely expired. Exiting thetiming of the time slot communication method in this way can beadvantageous, since a tag in a state where it listens for signals mustremain in its active state for relative long timespans by comparison toits timing in the time slot communication method, which negativelyaffects its energy balance, and leads to a shortened service life. Thefaster the search begins and the faster the search is again concluded,i.e., the normal mode in the time slot communication method is assumed,the more energy efficiently the system of tags operates. However, thetime for transmitting the locating signal can also be defined by thetime slot already being used in the normal mode, which is provided forcommunicating with the respective tag. As opposed to the normal mode,this time slot does not involve searching for signals transmitted viathe access point, but rather transmitting the locating signal.Regardless of whether the time slot communication method is retained orexited, transmission of the locating signal can also be started at anypoint in time, in particular one to be defined in advance.

The definition can also call for the locating signal to be transmittedmore frequently than just one time, for example to have several locatingsignals from one and the same tag available for further processing. Thispermits a statistical evaluation of the respective locating signal,which can improve the accuracy of position localization. However, thiscan also ensure that there will be an elevated probability of receivingthe locating signal at least one time in the event of disturbancesduring signal transmission. For example, transmission can take placewithin a defined timespan at random times autonomously determined by thetag. However, transmission can also take place at fixedly (pre)settimes.

In another aspect of the invention, the tag intended for receiving thelocating signal, in particular the tag whose position is unknown, isswitched by a received locating command from its normal mode into alocating mode, wherein a reception readiness for receiving locatingsignals exists in the locating mode for a locating signal receptionduration. Switching takes place by decoding the locating command bymeans of the logic stage. By comparison to the regular receptionduration of a time slot according to the time slot communication method,the tag can here assume a lengthened reception duration in the normalmode, and hence extricate itself from the time slot communication methodcharacteristic for the normal mode. In contrast to the normal mode, theaffected tag now also lies outside of the time slot intended forcommunicating with the access point in its active state, and can receivethe locating signals of the other tags in the aforesaid lengthenedreception duration. The length of the locating signal duration can befixedly predefined or respectively adjusted by the access point for thesearch in question. However, the expiration of the locating signalreception duration can also be indicated at an appropriate time by acommand or status message generated by the access point or one of thetags. The time at which the locating signal reception duration startscan correspond to the time at which the transmission of the locatingsignal starts, just as explained above, but also be set for an earlieror later point in time.

It has proven especially advantageous to design the locating signalreception duration so that all radio tags whose position is known andthat are involved in the search for the radio tag whose position isunknown can transmit their locating signal at least one time within thelocating signal reception duration, but preferably several times. Anadjustable value for the locating signal reception duration can betransmitted to the tag whose position is unknown for determining thelocating signal reception duration already with the transmission of thelocating command. The time slot communication method is preferablyexited at the start of the locating signal reception duration. The setlocating signal duration can have any value desired, but can preferablybe a multiple of the duration of a time slot of the time slotcommunication method in order to enable as quick a re-synchronization ofthe system as possible upon expiration of the locating signal receptionduration. After the locating signal reception duration has started, alltags assigned with transmitting the locating signal transmit theirlocating signal at times determined by them at the previously definedfrequency. For example, the plan for a specific tag may call fortransmitting N times (e.g., times) in the locating signal receptionduration, and the respective tag distributes the appearance of therespective locating signal randomly within the available timespan of thelocating signal reception duration. In the receiving tag, this leads toa statistical distribution of received locating signals and accompanyingreception qualities, and allows the receiving tag to recognize andeliminate obviously unusable locating signals.

Once the locating signal reception duration has expired, all tagsparticipating in the search re-synchronize themselves with the time slotcommunication method once again.

To allow the received locating signals to be used for determining theposition, it is advantageous that the radio tag intended for receivingthe locating signal, in particular the radio tag whose position isunknown, store the data pair for each locating signal received duringthe locating signal reception duration for later use. For example, thestored data pairs can be sorted in descending order of determinedreception quality for further use. If permitted by the computing powerof the logic circuit, this can already take place successively in thelocating signal reception duration while receiving a new locatingsignal, or otherwise only upon expiration of aforesaid locating signalreception duration or even at a later point in time, outsourced to thedata processing device.

In order to ensure that all receivable locating signals are accumulatedas quickly, efficiently and uninterruptedly as possible, it isadvantageous that the radio tag intended for receiving the locatingsignal, in particular the radio tag whose position is unknown, onlytransmit one or more stored data pairs after the locating signalreception duration has expired, wherein these data pairs arecommunicated via the access point to the data processing device, wherefinal locating can take place. The data pairs can be transmitted withina predefined transmission duration, which can be part of implementingthe locating mode. Just as the locating signal reception duration, thetransmission duration can measure a multiple of the duration of a timeslot for a communication in the normal mode. This case can also beimplemented in the search mode, so that no disturbances caused by radiosignals or collisions with radio signals of tags whose position is knownarise during radio communication with the access point, i.e., the tagswhose position is known behave calmly and transmit no signals. Onlyafter data pair transmission is complete can both the search mode andlocating mode be exited, and the normal mode be resumed. On the otherhand, the normal mode can be resumed for all participating tags afterthe complete transmission of all locating signals, or in other wordsafter the expiration of the locating signal reception duration andre-synchronization for the tags participating in the search, with thedata pairs being transmitted to the data processing device in the normalmode and in the respective predetermined time slot. The fastest possiblerestoration of normal operating conditions is ensured for allparticipating tags in this variant.

In another aspect of the invention, the radio tag whose position isunknown exhibits an indicating device, wherein the latter is used tovisualize status information, which represents its internal orsystem-wide status as a tag whose position is unknown that is to belocated. In this embodiment, for example, the tag can be a constituentof an electronic price-indicating sign (also referred as “electronicshelf label”, abbreviated ESL). In the normal mode, the indicatingdevice is used to display product prices or additional informationrelating to the product. As soon as the logic stage of the tag changesits operating state from the normal mode to the locating mode inresponse to the received locating command, this is visualized by meansof the logic stage using the indicating device in the form of statusinformation, in particular which a person can read. For example, thedisplay of the indicating device can read as follows: “Locating modeactive”. In addition, instructions to people can be visualized, e.g., “Iam wanted, please bring me to the cashier!”. For cases in which thestatus information is not to be readable for a person, a so-called QRcode can also be used, which is to be automatically processed, forexample by an optical reader. The indicating device can be realized withLCD technology, for example, but preferably also with electronic inktechnology (also referred to as E-ink as a synonym for electronicpaper).

In particular preliminarily, e.g., before transmitting the searchcommand and/or locating command, it has proven especially advantageousfor the radio tags to perform a calibration communication with an accesspoint whose position is known, and to which the radio tags areallocated, wherein the reception quality is determined for each involvedradio tag, and a correlation is defined between the determined receptionquality and the position for the respective radio tag whose position isknown. This improves the position localization by means of locatingsignals received from the tag whose position is unknown, since thereception quality depends heavily on the environment of the respectivetag. For example, ranges of up to 100 meters are possible in an openfield, while a range of approx. meters is virtually impossible to exceedon a business premises due to reflections and/or screening.

In another aspect of the invention, the tags communicate with an accesspoint in a normal mode using a time slot communication method, wherein,according to this time slot communication method, a defined number oftime slots is available in a defined time unit, and one or more radiotags is/are allocated to one of the time slots, and each radio tag canbe individually addressed in its time slot, so as to receive data orcommands from the access point and/or transmit data to the access point,wherein the tag whose position is unknown and/or at least one tag whoseposition is known exit(s) the time slot communication method so as totransmit or receive the locating signal. Exiting the time slotcommunication method means that the affected tag now follows anothertiming. As a consequence, the locating signal can be received ortransmitted in another time slot to which the affected tag is notallocated in the normal mode. In like manner, the systematics of thetime slots can be abandoned completely, and transmission or receptioncan take place within a timespan that no longer corresponds to a timeslot, e.g., lasts significantly longer. This measure contributes tosystem efficiency, since the search process can be executed relativelyquickly, and, liberated from the timing straightjacket imposed by thetime slot communication method, as many radio tags as possible canparticipate virtually simultaneously in a swarm in searching for themissed radio tag.

After the locating signal has been transmitted or received, the affectedradio tags again join the time slot communication method, and return tothe normal mode. After rejoining, the tags are again available forcommunicating with the access point in each time slot allocated to theaffected tag. Joining can take place immediately after the searchprocess has concluded, or at a later point in time. Upon conclusion ofthe transmission and reception sequence, the radio tags can remain inthe sleep state until the next time slot cycle starts, wake up shortlybeforehand, assume the active state, wait in the active state untilreceiving their synchronization signal, which they use to identify theallocated time slot, and return back to the normal mode viare-synchronization. In the event the tags have exited the timing of thetime slot communication method in order to transmit or receive thelocating signal, it is advantageous that the tags be designed toindependently calculate the earliest possible time for joining. Thefollowing is known in the system of tags when exiting the time slotcommunication method:

-   -   The duration D1 of the synchronization cycle (e.g., 15 seconds        for all tags).    -   The time D2 at which the time slot allocated to the respective        tag starts (e.g., 6.3 seconds for tag x).    -   The starting time D3 of the locating signal reception duration        in the synchronization cycle (e.g., 13 seconds for all tags).    -   The duration D4 of the locating signal reception duration (e.g.,        3 seconds for tag x).    -   The duration D5 of the lead time selected for waking up the        respective (each) tag from the sleep state on time prior to the        appearance of the time slot to which the tag is allocated (e.g.,        1 second for all tags).

These five parameters D1-D5 can be used to calculate the optimal,specifically the next, possible point in time, so as to wake up theaffected tag after the end of the locating signal reception duration ina timely fashion prior to the appearance of the next time slot to whichthe respective tag is allocated, and re-synchronize it with the timingof the time slot communication method again by receiving thesynchronization signal structure. Formula 1 below is used to calculatethe retention time D6 in the sleep state upon expiration of the locatingsignal reception duration.D6=D1−((D3−D2)+D4)−D5  Formula 1:

In the present example, a value of 4.3 seconds is calculated for D6.Since in this example the locating signal reception duration starts atsecond 13 in the first synchronization cycle and the duration D4 of thelocating signal reception duration measures 3 seconds, the locatingsignal reception duration extends for 1 second into the next (second)synchronization cycle. Parameter D6 now affirms that one must wait for4.3 seconds after the first second of the next (second) synchronizationcycle, i.e., until the absolute time stamp reaches 5.3 seconds in thissynchronization cycle, until the affected tag x awakes from the sleepstate. This tag x thus wakes up 1 second prior to the appearance of thenext possible time slot relevant to it. In analogy to the firstsynchronization cycle, the relevant time slot starts at the absolutetime stamp of 6.3 seconds in the second synchronization cycle. However,the correct wakeup time need not measure 1 second, but can rather alsoassume higher or lower values depending on permissible system limits,e.g., 0.5 or 0.2 or even 0.1 seconds. In addition, the advantage torejoining the timing of the time slot communication method as quickly aspossible is that no escalation takes place in the drift of the internaltime base for the tags participating in the search, as well as in thedrift of the tags which, while allocated to the access point triggeringthe search, do not actively participate in the search, but rather remainin the sleep mode during the search.

In another aspect of the invention, a data processing device is used toselect aforesaid group of radio tags from the totality of tags in asystem. Since a number of tags is always allocated to a specific accesspoint, a reduction to a group of radio tags having a lower number thanthe number of tags allocated to the access point can increase both theefficiency and effectiveness of the search. In addition, this measure isalso extremely energy efficient for the non-participating tags.

It has proven especially advantageous for the radio tag to exhibit asensor for emitting a sensor signal indicating that the radio tag wasremoved from another object, in particular a shelf, and for the logicstage to be designed to emit a status message about the radiocommunication stage if the sensor signal is present, wherein the statusmessage represents the detected removal from the other object.Configuring the tag in this way allows the tag to itself start themethod for locating its own position automatically. The status messagecan here be communicated to the access point to which the tag isallocated in the next time slot allocated to the tag, and the processfor locating the radio tag whose position is unknown can be startedthere autonomously or after coordination with the server responsible formanaging the system. The sensor can be realized using a stylus (pin),whose penetration depth into a housing of the tag was automaticallydetermined, e.g., with a contact that is either closed or open. However,the sensor can also detect the loss of contact with the object in acapacitive, inductive or any other suitable manner. For example, theappearance of the sensor signal can be mechanically or electronicallystored, so as to retain it until the next time the tag wakes up.However, the sensor signal can also be used to proactively wake up thetag. These measures sustainably prevent the loss of contact from beingmanipulated once detected.

In another aspect of the invention, the system can be designed in such away that, once a radio tag whose position is unknown has been located, anumber of radio tags whose position is known and which are positionedadjacent to the located radio tag are prompted to indicate the locationof the radio tag whose position is unknown in their environment by meansof their indicating device, e.g., by displaying a symbol, such as acircle, or a blinking behavior, etc. As a consequence, this selectedgroup of radio tags whose position is known can be used on the one handto visualize the system status with regard to the conclusion of locatingthe radio tags whose position is unknown, and on the other hand, giventheir proximity to the radio tag whose position is unknown, tofacilitate the manual searching process. With their indicating device,the selected radio tags whose position is known make it easier for asearching person to narrow down the area in which the radio tag whoseposition is unknown can be found. As a consequence, the searching personis guided to the target area.

This and other aspects of the invention may be gleaned from the figuresdiscussed below.

BRIEF DESCRIPTION OF FIGURES

The invention will be explained once again in more detail below drawingreference to the attached figures and based upon exemplary embodiments,with the invention not being limited thereto. The same components arehere provided with identical reference numbers on the various figures.Schematically shown on:

FIG. 1 is a system according to the invention;

FIG. 2 is a block diagram of a radio tag according to the invention;

FIG. 3 is a flowchart for a method according to the invention;

FIG. 4 is a data structure according to a first exemplary embodiment forlocating a radio tag;

FIG. 5 is a state diagram for tags according to a second exemplaryembodiment of the method;

FIG. 6 is a data structure according to a second exemplary embodimentfor locating a radio tag;

FIG. 7 is a first application of the invention;

FIG. 8 is a second application of the invention;

FIG. 9 is a third application of the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Shown on FIG. 1 is a system 4 in a business premises 1, which is dividedinto a showroom 2 and an administrative office 3. The system 4 exhibitsa group of radio tags, abbreviated tags, T1-T24. Each tag Tn stores anidentifier K1-K24 unique to the respective tag Tn in the form of data(not visualized on FIG. 2, but see FIG. 2), so as to clearly identifyit. Each tag Tn forms a constituent of the electronic price-indicatingsign E1-E24. Each price-indicating sign En exhibits an indicating deviceA1-A24. Each price-indicating sign En except for the seventeenthprice-indicating sign E17 is fastened to an attachment rail of a shelfR1-R5. Each shelf Rn has individual products P1-P24, and the indicatingdevices An display the respective prices for the products Pn. Theseventeenth price-indicating sign E17 was removed from its planned,seventeenth space (denoted by arrow Y), and is located on the floor nearthe fifth shelf R5. The system 4 further exhibits a server 5 thatcomprises a data processing device and has processing means 6 as well asstorage means 7. The system 4 further exhibits two access points X1 andX2, which are connected with the server 5 by a hardwired network 8.

Before the tags Tn can be used in the system 4, they must be manuallysecured to the positions provided for them on the shelves Rn, andregister to the system 4. The tags Tn here receive radio signals fromthe access points Xn, and, as a function of the reception quality of theradio signals, register to the access point Xn for which the bestreception quality is present. In the case at hand, let it be assumedthat the Tags T1-T6, T10-T12 and T16-T18 are allocated to the firstaccess point X1, and the tags T7-T9, T13-T15 and T19-T24 are allocatedto the second access point X2. The communication by which prices orother information relevant to the customer are communicated by theaccess point Xn to the tags Tn in their normal mode takes place by meansof a time slot communication method. Within n seconds, m time slots arehere used. Each of the tags Tn is allocated to a time slot, whereinseveral tags Tn can also be allocated to a specific time slot. The tagsTn are in their sleep state outside of their time slot, and in theiractive state just before their time slot and while in their time slot.By waking up prior to the time slot in question on time, they can keeptheir internal clock synchronous with the time grid of the time slotcommunication method, and thus reliably transmit data in their time slotwith the access point Xn, to the extent required by the access point Xn.

A planogram is stored in digital form 1 with the storage means 6. Aplanogram is a schematic drawing of a retail store device thatillustrates product placements. As a result, the stored planogramrepresents the relationship between a shelf location, a product, an EANcode and an identifier Kn of the respectively allocated tag Tn of theelectronic price-indicating sign in the form of a digital list or adatabase. For example, this relationship can be established by means ofwearable EAN code readers, with which the EAN code of a product Pn canbe read in, transmitted to the server 5, and allocated to a tag Tn atthe server 5.

The tags Tn all have an identical design. FIG. 2 visualizes thestructure of one of the tags Tn. The tag Tn comprises a logic stage 9, aradio communication stage 10 and an indicating device 11. The radiocommunication stage 10 realizes the analog interface, with which radiosignals can be exchanged with the access points Xn. Correspondingreceiving and/or transmitting data RXD, TXD are communicated between theradio communication stage 10 and logic stage 9.

The logic stage 9 processes data and commands, in the present case inparticular a search command and a locating command, and apart from anormal mode also provides a search mode and locating mode. Once thesearch command has been received, a switch is made from the normal modeto the search mode. Once the locating command has been received, aswitch is made from the normal mode to the locating mode. In the searchmode, a locating signal On that comprises the identifier Kn of therespective tag Tn is generated, and transmitted as a radio signal in thetime slot allocated to the respective tag Tn. A switch is then made fromthe search mode to the normal mode. In the locating mode, a receptionreadiness for receiving locating signals On during a locating signalreception duration is established, which is present as soon as thelocating signal is transmitted. Each received locating signal On isevaluated for its reception quality Qn and the identifier Kn transmittedwith the locating signal On, and a data pair Dn consisting of theidentifier Kn and reception quality Qn is stored. After the locatingsignal reception duration has expired, the stored data pairs are rankedin descending order of reception quality, and communicated by radiosignals to the server 5, where further evaluation takes place.

The indicating device 11 is based on extremely energy-saving electronicink technology (also referred to as E-ink as a synonym for electronicpaper), and is initiated by the logic stage 9 with display data AD,which are converted into visually perceivable information.

In reference to FIG. 1, let it now be assumed that the seventeenthprice-indicating sign E17 was removed from its planned, seventeenth spot(denoted by part Y), and is now located on the floor, but still inproximity to the fourth shelf R4, and also within communication range ofthe first access point X1. As part of a routine visual inspection of theprice-indicating signs En, an employee noticed that the seventeenthprice-indicating sign is missing. He or she uses their EAN scanner toscan the EAN product code of the product P17, transmits the EAN productcode via a conventional WLAN infrastructure (not shown; aninfrastructure differing from the communication infrastructure of theaccess point) of the business premises 1 to the server 5, and starts asearch query at the server 5. The seventeenth tag T17 is identified as atag to be searched at the server 5. Since a communications link stillremains between the seventeenth tag T17 and first access point X1, thesearch to be started by the server 5 is confined to involving tagsT1-T6, T10-T12 and T16-T18. A process 12 for locating the seventeenthtag T17 is now started for this partial group of tags Tn, whereinreference is made below to the flowchart according to FIG. 3.

The process 12 is started in a block 13, and it is assumed that all tagsTn are in the normal mode. The locating command is sent to theseventeenth tag T17 in a block 14, and this takes place in theappropriate time slot for communicating with the seventeenth tag T17.This causes a switch from the normal mode to the locating mode at tagT17. The tag 17 in the locating mode visualizes its status by means ofthe indicating device A17, wherein “locating mode active” is displayedas status information. The search command is sent to the tags T1-T6,T10-T12, T16 and T18 in a block 15, and this takes place in theappropriate time slot for the respective tag T1-T6, T10-T12, T16 andT18. This causes a switch from the normal mode to the search mode attags T1-T6, T10-T12, T16 and T18. Since the case at hand involves a timeslot communication method, the blocks 14 and 15 on FIG. 3 illustratemore of a logic state, and do not necessarily indicate the chronologicalsequence when switching from the normal mode to the locating mode orsearch mode.

Let it be assumed below that the respective tag Tn transmits thelocating signal strictly according to the time grid of the time slotcommunication method. Only the seventeenth tag T17 to be searched for isin the locating mode during the entire number of time slots. Thetransmission of locating signals by the participating tags starts afterall participating tags have received and processed their respectivecommand (locating command, search command) with the beginning of thenext time slot cycle.

In a block 16, the first locating signal O1 is transmitted by the firsttag T1 being operated in the search mode in the time slot allocatedthereto. In a block 17, the first locating signal O1—to the extentphysically possible—is received by the tag T17 being operated in thelocating mode, evaluated with respect to the first reception quality Q1and identifier K1, and a first data pair D1 comprised of the receptionquality Q1 and identifier K1 is stored. In block 18, a check isperformed at tag 17 as to whether the locating signal reception durationhas already expired. Should this not be the case, the process 12branches to block 16, wherein the next locating signal O2 is transmittedby the tag T2 in the time slot allocated thereto. In block 17, thissecond locating signal O2—to the extent physically possible—is receivedby the tag T17 being operated in the locating mode, evaluated withrespect to the second reception quality Q2 and identifier K2, and asecond data pair D2 comprised of the reception quality Q2 and identifierK2 is stored. A loop 19 formed by blocks 16, 17 and 18 is run through asoften as it takes for all tags T1-T6, T10-T12, T16 and T18 participatingin the search for the tag T17 being operated in the locating mode totransmit their respective locating signal O1-O6, O10-O12, O16 and O18 inthe respective time slots allocated to them, or in other words until thelocating signal reception duration has expired. In the case at hand, itwas determined that the locating signal reception time measuresprecisely one time slot cycle. Since the time grid of the time slotcommunication method was used in the present case, no re-synchronizationis necessary at least for the tags T1-T6, T10-T12, T16, T18 thattransmit the locating signal. Only tag T17 must re-synchronize itself.

However, the locating signal reception duration can also extend overseveral time slot cycles.

As explained above, visualizing in the form of blocks 16, 17 and 18 inthis conjunction is geared more toward illustrating how the method islogically structured rather than reflecting the chronological sequences.

The list of data pairs Dn stored in the seventeenth tag T17 forms a datastructure DS for determining or narrowing down the position of the tagT17 that stores this data structure DS. The stored data structure DS isillustrated on FIG. 4. The data structure DS exhibits the identifiers Knin a first column, and the values representing the reception quality En,e.g., the RSSI value, in another column. The respective value of theRSSI is lower for tags Tn lying further away in relation to the positionof the tag T17 and for more heavily screened tags Tn (e.g., tags hiddenbehind shelves and products), and higher for tags Tn that are closer tothe tag 17 and/or less screened.

In further consequence, the seventeenth tag T17 transmits the storeddata pairs Dn to the server 5. Depending on the number of stored datapairs Dn, this takes place in a single time slot allocated thereto, orin a number of such time slots appearing in a time-staggered manner.

In a block 21, the position of the seventeenth tag T17 whose position isunknown is determined or at least narrowed down based on the data pairsDn transmitted to the server and the known positions of the tags T1-T6,T10-T12, T16 and T18. By sorting the transmitted data structure DS basedon descending values of the RSSI, it is determined that thesearched/missing tag T17 must be located in an area between tag T6 andT12, but to the right of tag T12 and closer to tag T12 than to tag T6.

The position of the seventeenth tag T17 determined in this way isreported, i.e., disclosed, to an employee, e.g., via the mobile EAN codereader, so that he or she can track down tag T17 in as targeted a manneras possible, and secure it to its planned spot on the shelf 4 in thearea of the product P17. Once there affixed, the employee can use his orher EAN code reader, for example, to initiate a calibration sequence, inwhich the communication parameters relevant for communicating with theseventeenth tag 17 (e.g., RSSI and/or link quality) can be acquired forhis or her desired position.

The status information “locating mode active” provided by the indicatingdevice can accelerate the manual process of tracking down, specificallyin instances where the electronic price-indicating sign E17 wasconsciously, i.e., deliberately, or accidently fastened to a shelfposition not allocated to it by plan. In addition, the provided statusinformation (“locating mode active” without any other product data orprice information) ensures that a customer will not be misled by suchwrongly positioned price-indicating signs. This display of statusinformation can also be retained in the already resumed normal mode.

The measures according to the invention discussed in detail above canalso be used for automatically verifying the position of individual tagsTn. The server 5 knows the desired position of each tag Tn in thebusiness premises 1 visualized on FIG. 1, and hence also the proximityor distance of the individual tags Tn relative to each other. The server5 automatically switches a respective one of the tags Tx (with x=1 . . .m; m=total number of tags) to the locating mode and all other tags Tn(n=1 . . . m without x) to the search mode, and, after implementing theprocess for locating the tags Tx, receives and processes the respectivedata structure DS. The obtained data structure DS is used at the server5 to check whether the tag Tx is localized at its desired position withsufficient probability. If the probability is insufficient, e.g.,because the deviation from the desired values for the reception qualityof the tags Tn is too high for an expected configuration of adjacenttags Tn, or basically an unexpected configuration of adjacent tags Tn isdetermined, the affected tag Tx can be tracked down at a later point intime by an employee, for example. This automatic verification ofposition for all tags Tn can be performed every weekday outside ofbusiness hours, for example periodically.

However, the tags Tn can also be designed to independently determinethat they were removed from a shelf Rn. This can take place by means ofa sensor (not shown, e.g., integrated into the electronicprice-indicating sign), which is coupled with the logic stage 9. As soonas the tag Tn in its normal mode switches from the sleep state to theactive state, the sensor status of the sensor is checked. If the sensorstatus indicates that the connection to the shelf Rn has been lost(e.g., at the time the active state is assumed or even before that), theaffected tag Tx reports this to the server 5 in the next time slotprovided for its communication. The process for locating the tag Txwhose position is unknown is then implemented for the affected tag Tx.The data structure DS obtained from the tag Tx is used for localizingthe tag Tx at the server 5, as discussed above.

However, the process for locating the tags Tx whose position is unknowncan also be automatically started by the server 5 even when the server 5determines that the tag Tx has reported to an access point Xn forcommunication, which based on its position should with a high degree ofprobability not even be available for a communication with the tag Tx atall.

However, the measures according to the invention discussed above canalso be used to track a moving tag Tx whose position is unknown. In thisapplication, a number of tags Tn whose position is known are placed inknown positions (e.g., at periodic intervals) in a room (e.g., fastenedto the floor or ceiling or built in there). The tag Tx whose position isunknown is fastened to a shopping cart, for example, which is moved by aperson through the business premises. In order to track the movement ofthe shopping cart, the process for locating the tag Tx is started atspecific times (e.g., every 10, 20, 30 or 60 seconds), and thenexecuted, so that, for example, a data structure DS is transmitted fromthe tag Tx to be located to the server in periodic intervals, where thetrajectory of the shopping cart, and ultimately of the person, isavailable based upon the configuration of the immediate neighbors oftags Tn, which changes during the movement. If the tag Tx whose positionis unknown is coupled with an indicating device similarly to one of theelectronic price-indicating signs, location or rather product-relatedinformation can be presented within the context of the respectivelydetermined position, in particular during a movement in a businesspremises. An expansion of this application can also provide that the tagTx whose position is unknown and which is fastened to the shopping cartexhibit an interface for communicating with the other mobile device, forexample a mobile phone or smart phone of the person. A purchase list canbe transmitted to the tag Tx via this interface, which consequently ismade available to the server 5 via the respective access point Xn. Basedon the determined position or movement of the tag Tx, the server 5 canprovide navigation information on the indicating device coupled with thetag Tx, and in this way support the person in transacting the purchases.

FIG. 5 explains another embodiment of the method described inconjunction with FIG. 1 in the form of a state diagram. The time t isrecorded along the abscissa in seconds for each of the four time slotcycles Z1 to Z4, wherein each time slot cycle lasts for seconds. Withina time slot cycle Z1-Z4, for example, 255 time slots are available persecond for communicating with the access point X1 (see FIG. 1). The tagsTx participating in the search are recorded on the ordinate. The statesor modes for the individual tags Tx are recorded in chronologicalsequence from left to right in the abscissa direction.

The embodiment discussed here can also be applied to the systemexplained according to FIG. 1 and the other exemplary applications andexemplary embodiments described in detail. In contrast to theexplanations for FIG. 1, however, the timing of the time slotcommunication method used in the normal mode is exited in the presentcase. This approach has proven to be faster, more efficient, and consumeless energy, and will be described in detail based on FIG. 5.

For reasons of clarity, it was decided not to illustrate the time slotsthemselves (e.g., 255 per time slot cycle). Within the first time slotcycle Z1, the tags T1-T6, T10-T12, T16 and T18 allocated to the firstaccess point X1 receive the search command SB in the respective timeslots allocated thereto, and the tag 17 receives the locating commandOB. As also shown on FIG. 5, several of the tags T1-T6, T10-T12, T16-T18(collectively referred to as Tn) can be allocated to one and the sametime slot (e.g., see: T1 and T2). The consecutive numeration using thenumbers one to eighteen divulges nothing about the allocation of thetags T1-T18 to one of the time slots, e.g., which can take place via adistinct address of the respective tag Tn, such as the Mac address(hardware address). The allocation was determined in advance, and isknown to the access point X1. Before receiving the respective commandSB, OB, the tags Tn in the normal mode NM are in the sleep state SZ.While receiving the commands SB, OB, the tags Tn are in the active stateAZ. After processing the command SB, OB, the tags T1-T6, T10-T12, T16and T18 are in the search mode SM, and tag T17 is in the locating modeOM.

Within the framework of this communication (delivery of commands SB,OB), the tags Tn are also notified about the time slot or absolute timestamp in the first time slot cycle Z1 at which the search for the tagT17 whose position is unknown is to be started. In the case at hand, thetime stamp of 13 seconds has been set as the starting time for thelocating signal reception duration OED in the first time slot cycle. Thelocating signal reception duration OED was set at 3 seconds, and thusextends by one second into the second time slot cycle Z2. However, thisinformation can also be transmitted in advance to prepare for the searchin a time slot cycle (not shown) preceding the first time slot cycle Z1.

Within the framework of delivering the commands SB and OB, the tagsT1-T16 and T18 are also notified that each of them must transmit fiftylocating signals within the locating signal reception duration OED. Thiscan also be disclosed to the tag T17 whose position is unknown, but doesnot have to be.

After receiving the commands SB, OB, the tags Tn again return to theirsleep state SZ, and remain in the sleep state SZ until starting time ZS.In contrast to the normal timing of the time slot communication method,their timing now proceeds differently than in the normal mode NM, inwhich they would wait for the next time slot allocated thereto in thesecond time slot cycle Z2. In the modified timing, the tags T1-T18 nowwait for the starting time ZS, which in the case at hand is assumed tocome 2 seconds before the first time slot cycle Z1 ends (absolute timestamp 13 seconds in the first time slot cycle). The search for theseventeenth tag T17 whose position is unknown begins at this startingpoint ZS.

At the starting point ZS, the tags T1-T6, T10-T12, T16 and T18 whoseposition is known have activated the search mode, and the tag T17 whoseposition is unknown has activated the locating mode. Shortly (approx.200 milliseconds) before the starting point ZS, they exit their sleepstate SZ and move into an active state SAZ specific to their respectivemode (search mode SM, locating mode OM). Knowing the time slot of thetime slot communication method allocated to them in the normal mode andthe starting point ZS received beforehand, the affected tags Tncalculate the wakeup moment themselves.

In the specific active state SAZ, the tags T1-T6, T10-T12, T16 and T18whose position is known transmit their fifty locating signals at randomtimes they themselves defined during the locating signal receptionduration OED of three seconds. As a consequence, when the locatingsignal OS arises is randomly distributed within the locating signalreception duration OED.

In the specific active state SAZ, the tag T17 whose position is unknownis permanently on reception, and, provided no collisions between signalsare present, it receives a series of locating signals OS transmitted bythe tags Tx whose position is known, evaluates the latter with respectto the reception quality Qx and identifier Kx of the respectivelytransmitting tags Tx whose position is known, and stores the respectivedata pair Dx. Several data pairs Dx allocated to a specific tag Tx canhere also be stored, as depicted on FIG. 6. Recorded on FIG. 6 for thefirst fifteen of a series of reception events (first column labeled #numbers the reception events) are the identifiers Kx of the transmittingtag Tx, as well as the value of the accompanying received signalstrength indicator RSSIx and the value of the link quality LQx values.

Upon expiration of the locating signal reception duration OED, allparticipating tags T1-T18 return to the sleep state SZ again, which forreasons of clarity is only visualized for the first tag T1 on FIG. 5.They wake up from this sleep state SZ in a timely manner, e.g., 1 secondbefore the appearance of the next time slot to which the respective tagTn is allocated. This is denoted on FIG. 5 by a lengthened duration ofthe active state for each of the tags Tn by comparison to the first timeslot cycle Z1. Since the tags Tn have been informed about all parametersof the time slot communication method and about their exit from thetiming of the time slot communication method, the tags Tn participatingin the search independently calculate their wakeup times (see formula 1in the general portion of the specification). After waking up, theyremain in the active state AZ and listen for the signals of the accesspoint X1, until they receive the synchronization signal structureintended for them and reenter the synchronous state (also referred to asre-synchronize). Following their re-synchronization (i.e., reentry intothe time slot communication method), the tags Tn that previouslyparticipated in the search are again available in the time slotcommunication method for communicating with the access point X1. Asdepicted on FIG. 5 in the third or fourth time slot cycles Z3 and Z4,the duration of their active state AZ following re-synchronization isnow as long as it was in the first time slot cycle Z1, before the searchwas performed for the tag T17 whose position is unknown.

This embodiment of the invention is relatively energy efficient, sinceall of the tags Tn participating in the search are together only activein their specific active state SAZ during a limited timespan, and remainin the sleep state SZ before and after. System efficiency is also hardlyinfluenced by the search, because the search can be confined to ashorter duration (locating signal reception duration OED) by comparisonto the duration of a time slot cycle, after which all tags Tnparticipating in the search independently reenter the time slotcommunication method again as quickly as possible.

In another exemplary embodiment, the starting time ZS can also coincidedirectly with the time slot, which directly follows the time slot inwhich the command SB or OB is delivered to the last tag to participatein the search (here tag T18). In this case, there is no need for theaffected tag (here T18) to reenter the sleep state after receiving thecommand. It can switch directly into the respective mode (search mode orlocating mode). In addition, the locating signal reception duration OEDcan be dimensioned in such a way that the search already concludeswithin the first time slot cycle Z1. In this case, re-synchronizationcan already be started at the beginning of the second time slot cycleZ2. It can be especially advantageous for the end of the locating signalreception duration to coincide with the end of the first time slot cycleZ1. In this case as well, those tags Tn participating in the searchwhose time slot lies in proximity to or at the start of the time slotcycle Z2 do not absolutely have to enter the sleep state SZ. All ofthese variants help to accelerate the search, and also contribute tosystem efficiency and energy savings.

In another implementation, the tags participating in the search (buteven all tags not participating therein which are allocated to the sameaccess point as the tags participating in the search) switch to thesleep mode in each instance after the locating signal reception durationOED has expired, and only wake up shortly before the appearance of therespective time slot to which they are allocated in the next or nextplus one time slot cycle (as measured upon expiration of the locatingsignal reception duration OED). They can also wake up collectively at apoint in time defined in advance, e.g., after expiration of the locatingsignal reception duration OED at the start of the next time slot cycle,and thereafter wait in the active state for the appearance of the timeslot to which they are allocated, so as to re-synchronize themselveswith the latter.

The following three FIGS. 7 to 9 summarize three applications of theinvention in a highly schematized manner. The number of radio tags Tnparticipating in the search was reduced to a respective five for reasonsof clarity, wherein the seventeenth radio tag T17 assumes the role of aradio tag whose position is unknown, and the other radio tags T1, T7,T10 and T18, or T1-18 for short, assume the role of radio tags whoseposition is known.

In the first application according to FIG. 7, the access point X1 isused within the framework of a communication according to the time slotcommunication method to move the radio tag T17 whose position is unknownfrom the normal mode to the search mode, and the other radio tags T1-18whose position is known from the normal mode to the locating mode. Allinvolved radio tags T1-18, T17 then wait for the locating signalreception duration OED to start. The tag T17 whose position is unknowntransmits one or more locating signals On in the locating signalreception duration OED, as denoted by arrows marked with referencenumber S1. The radio tags T1-18 whose position is known receive thelocating signals On and evaluate them with respect to reception qualityQn (Q1, Q7, Q18, Q10). After the locating signal reception duration OEDhas expired and all radio tags T1-18 have been re-synchronized with thetime slot communication method, the reception quality Qn determined atthe respective radio tag T1-18 whose position is known is transmitted tothe access point X1, as denoted by arrows marked with reference numberS2, wherein the access point X1 knows the respective radio tag T1-18whose position is known or its identifier Kn (K1, K7, K10, K18). Theaccess point X1 subsequently takes the reception quality Qn obtained bythe respective radio tag T1-18 along with the identifier Kn known to itof the respective radio tag T1-18 whose position is known and relaysthem to evaluation.

In the second application according to FIG. 8, the access point X1 wasused within the framework of a communication according to the time slotcommunication method to move the radio tag T17 whose position is unknownfrom the normal mode to the locating mode, and the other radio tagsT1-18 whose position is known from the normal mode to the search mode.All involved radio tags T1-18, T17 then wait for the locating signalreception duration OED to start. The radio tags T1-18 whose position isknown each transmit their locating signal On to the radio tag T17 whoseposition is unknown in the locating signal reception duration OED, asdenoted by arrows marked with reference number S1. The respectivelocating signal On is received at the radio tag T17 whose position isunknown and evaluated. As a result of the evaluation, the radio tag T17whose position is unknown stores data pairs comprised of the identifierKn of the respective radio tag T1-T18 and reception quality Qn. Afterthe locating signal reception duration OED has expired and all radiotags T1-18 have been re-synchronized with the time slot communicationmethod, the data pairs are transmitted to the access point X1, asdenoted by the arrow marked with reference number S2, and from thererelayed to evaluation.

As opposed to the second application, the locating signals Ontransmitted in the locating signal reception duration OED are in thethird application according to FIG. 9 addressed to the access point X1,while the radio tag whose position is unknown receives the locatingsignals On, which is denoted by the reference number S3 between theradio tag T17 whose position is unknown and the respective arrow S1, andevaluates them. The determined and stored data pairs DP are transmittedto the access point X1 from the radio tag T17 whose position is unknown,and from there relayed to the data processing device.

Let it be mentioned that the locating signals On that were described asalways addressed in the three applications discussed above can also beused without being addressed.

Finally, let it be noted once again that the figures described in detailabove are only exemplary embodiments, which the expert can modify in awide variety of ways without departing from the area of the invention.For the sake of completeness, let it further be noted that the use ofindeterminate articles “a” or “an” does not rule out that the featuresin question can also be repeatedly present. Individually disclosedfeatures can also be combined with other features, without deviatingfrom the concept of the invention.

The invention claimed is:
 1. A method for locating a radio tag (T17)whose position is unknown, wherein, in a group of radio tags (T1-T18)that are allocated by radio to only a single access point, the radiotags (T1-T18) communicate with the access point in a normal mode using atime slot communication method, wherein, according to this time slotcommunication method, a defined number of time slots is available in adefined time unit, and one or more radio tags (T1-T18) are allocated toone of the time slots, and each radio tag (T1-T18) can be individuallyaddressed in its time slot, so as to receive data or commands from theaccess point and/or transmit data to the access point, a locating signal(On): a) is either transmitted by one or more radio tags (T1-T6,T10-T12, T16, T18) whose position is known and received by the radio tag(T17) whose position is unknown, b) or transmitted by the radio tagwhose position is unknown (T17) and received by one or more radio tags(T1-T6, T10-T12, T16, T18) whose position is known, and, in both casesa) and b), the reception quality (Qn) of the locating signal (On) isdetermined and provided at the radio tag (T1-T6, T10-T12, T16, T18; T17)receiving the locating signal (On) in order to narrow down the positionof the radio tag (T17) whose position is unknown wherein the radio tag(T17) whose position is unknown and/or at least one radio tag (T1-T6,T10-T12, T16, T18) whose position is known exit(s) the time slotcommunication method so as to transmit or receive the locating signal(On), and wherein, after the locating signal (On) has been transmittedor received, the affected radio tags (Tn) again join the time slotcommunication method, and return to the normal mode, wherein theaffected radio tags (Tn) independently calculate the earliest possibletime for joining.
 2. The method according to claim 1, wherein the radiotag (Tn) whose position is known and that receives the locating signal(On) transmits the evaluated reception quality (Qn) to a data processingdevice (5), and the data processing device (5) narrows down the positionof the radio tag (T17) whose position is unknown, knowing the positionof the radio tag(s) (T1-T6, T10-T12, T16, T18) whose position is knownand taking into account the reception quality (Qn) received by therespective radio tag (T1-T6, T10-T12, T16, T18) whose position is known.3. The method according to claim 1, wherein the radio tag (T1-T6,T10-T12, T16, T18) whose position is known that transmits the locatingsignal (On) transmits the locating signal (On) along with its identifier(Kn), and wherein the radio tag (T17) whose position is unknown thatreceives the locating signal (On) reevaluates the locating signal (On)with respect to the identifier (Kn) transmitted along with the locatingsignal (On), and provides a data pair (Dn) comprised of the identifier(Kn) and accompanying reception quality (Qn).
 4. The method according toclaim 3, wherein the radio tag (T17) whose position is known and thatreceives the locating signal (On) transmits the data pair (Dn) to a dataprocessing device (5), and the data processing device (5) narrows downthe position of the radio tag (T17) whose position is unknown, knowingthe position of the radio tag(s) (T1-T6, T10-T12, T16, T18) whoseposition is known and taking into account the data pair (Dn) received bythe respective radio tag (T17) whose position is unknown.
 5. The methodaccording to claim 1, wherein the tag (Tn) intended for transmitting thelocating signal (On), in particular the tag (T1-T6, T10-T12, T16, T18)whose position is known, is switched from its normal mode into a searchmode by a received search command, wherein the locating signal (On)and/or the time for transmitting the locating signal (On) and/or thefrequency of transmitting the locating signal (On) is determined in thesearch mode.
 6. The method according to claim 1, wherein the tag (Tn)intended for receiving the locating signal (On), in particular the tag(T17) whose position is unknown, is switched by a received locatingcommand from its normal mode into a locating mode, wherein a receptionreadiness for receiving locating signals (On) exists in the locatingmode for a locating signal reception duration (OED).
 7. The methodaccording to claim 6, wherein the locating signal reception duration(OED) is designed so that all radio tags (T1-T6, T10-T12, T16, T18)whose position is known and that are involved in the search for theradio tag (T17) whose position is unknown can transmit their locatingsignal (On) at least one time within the locating signal receptionduration (OED), but preferably several times.
 8. The method according toclaim 6, wherein the radio tag (Tn) intended for receiving the locatingsignal (On), in particular the radio tag (T17) whose position isunknown, stores a data pair (Dn), comprised of the identifier (Kn) andaccompanying reception quality (Qn), for each locating signal (On)received during the locating signal reception duration (OED) for lateruse.
 9. The method according to claim 8, wherein the radio tag (Tn)intended for receiving the locating signal, in particular the radio tag(T17) whose position is unknown, only transmits one or more stored datapairs (Dn) after the locating signal reception duration (OED) hasexpired.
 10. The method according to claim 1, wherein the radio tag(T17) whose position is unknown exhibits an indicating device, whereinthe latter is used to visualize status information, which represents itsinternal or system-wide status as a radio tag whose position is unknownthat is to be located.
 11. The method according to claim 1, wherein acalibration communication with an access point (X1, X2) whose positionis known is performed with the radio tags (Tn), wherein the receptionquality (Qn) is determined for each involved radio tag (Tn), and arelation is defined between the determined reception quality (Qn) andthe position for the respective radio tag (Tn) whose position is known.12. The method according to claim 1, wherein a data processing device isused to select aforesaid group of radio tags (T1-T18) from the totalityof tags (Tn) in a system, in particular from the totality of radio tagsallocated to a radio communication device, in particular the accesspoint.
 13. A system (4) for locating a radio tag (T17) whose position isunknown, comprising a group of radio tags (Tn) that are allocated byradio to only a single access point, wherein the radio tags (T1-T18)communicate with the access point in a normal mode using a time slotcommunication method, wherein, according to this time slot communicationmethod, a defined number of time slots is available in a defined timeunit, and one or more radio tags (T1-T18) are allocated to one of thetime slots, and each radio tag (T1-T18) can be individually addressed inits time slot, so as to receive data or commands from the access pointand/or transmit data to the access point, wherein a) either one or moreradio tag(s) (T1-T6, T10-T12, T16, T18) whose position is known is/aredesigned to transmit a locating signal (On) and the radio tag (T17)whose position is unknown is designed so that the locating signal (On)can be received as soon as the radio tag(s) (T1-T6, T10-T12, T16, T18)whose position is known have transmitted it, b) or the radio tag (T17)whose position is unknown is designed to transmit a locating signal(On), and one or more radio tag(s) (T1-T6, T10-T12, T16, T18) whoseposition is known are designed so that the locating signal (On) can bereceived as soon as the radio tag (T17) whose position is unknown hastransmitted it, and in both cases a) and b), the radio tag (T1-T6,T10-T12, T16, T18; T17) that receives the locating signal (On) isdesigned to evaluate the locating signal (On) with respect to thereception quality (Qn) and to provide the determined reception quality(Qn) as the basis for narrowing down the position of the radio tag (T17)whose position is unknown, wherein the radio tag (T17) whose position isunknown and/or at least one radio tag (T1-T6, T10-T12, T16, T18) whoseposition is known is/are designed to exit the time slot communicationmethod so as to transmit or receive the locating signal (On), andwherein the radio tags (T1-T18) are designed to again join the time slotcommunication method and return to the normal mode after the locatingsignal (On) has been transmitted or received, and wherein the radio tags(T1-T18) are designed to independently calculate the earliest possibletime for joining.
 14. The system according to claim 13, wherein eachradio tag (Tn) transmitting the locating signal (On) is designed togenerate and transmit a locating signal (On) that exhibits theidentifier (Kn) of the radio tag (Tn), and wherein each radio tag (Tn)receiving the locating signal (On) is designed to evaluate the receivedlocating signal (On) with respect to the identifier (Kn) transmittedwith the locating signal (On) and generate a data pair from theidentifier and accompanying reception quality (Qn).
 15. The system (4)according to one of claim 13, which exhibits a data processing device(5), wherein the data processing device (5) is designed to process adata pair received by a radio tag (T17) in such a way that, knowing theposition of the radio tag (T1-T6, T10-T12, T16, T18) belonging to therespective identifier (Kn) and taking into account the evaluatedreception quality (Qn), the position of the radio tag (T17) whoseposition is unknown can be narrowed down or determined.
 16. The system(4) according to one of claim 13, wherein the system, preferably anaccess point (X1, X2) of the system (4), more preferably a dataprocessing device (5) of the system (4), is designed to generateinformation for a located radio tag (T17) that is correlated with thedetermined location of the radio tag (T17), and the access point (X1),to which the located radio tag (T17) is allocated, is designed totransmit the information to the located radio tag (T17).